In semiconductor power amplifiers (PAs), which are used in a multitude of wireless devices such as cellular telephones, wireless portable devices and so forth, power amplification circuitry can be coupled to an output load by means of an output network. This output network can be in the form of a transformer having an input side coil and an output side coil. Conventionally, the input side coil is referred to as a primary coil and the output side coil is referred to as a secondary coil. These two coils are strongly coupled and have a coupling coefficient k therebetween.
As semiconductor technology advances, an output network including a transformer can be formed on a single semiconductor die. For example, the input side coil can be formed on a first metal layer formed on a substrate and the output side coil formed on a second metal layer formed on the substrate. Referring to FIG. 1A, which is a schematic representation of a prior art transformer, the input coil is referred to as port A and the output coil is referred to as port B. As seen in FIG. 1A, these two coils each have an inductance L and can be strongly coupled together. In a physical representation as shown in FIG. 1B, this transformer includes a first coil 10 that is formed on a first metal layer (e.g., M1) and a second coil 20 that is formed on a second metal layer (e.g., M2). The lines leading to the semi-circular portion of first coil 10 may be metal lines coupled to various circuitry, and similarly, the lines extending from second coil 20 may be metal lines coupled to other circuitry. The performance of this transformer structure, as represented by power efficiency, can improve as the metal layers thicken, up to a point. Eventually, the outer edges of the coils are too far apart and the coupling coefficient is reduced. At this point, a skin effect also reduces the amount of current flowing. Accordingly, techniques and structures to overcome such deficiencies are needed.